Day of the Daleks
Featuring: Third Doctor, Jo Grant, Brigadier Lethbridge-Stewart, Sergeant Benton, Mike Yates
The Doctor is experimenting with his dematerialisation circuit when he accidentally creates a freak effect where he and Jo meet themselves from later in their timestream. However, the Doctor is able to return things to normal fairly soon.
Sir Reginald Styles, a dipl;omat who is hostiug a world peace conference, is attacked by a guerilla. UNIT, who are provising security, investigate. Searching the grounds, they discover an injured guerilla, his weapon, and a black box. The Doctor discovers that the gun is an ultrasonic disintegrator and the box is a crude time machine. He starts fiddling with it, and the guerilla disappears, taken back to his own time.
When the guerillas next attack, the Doctor and Jo are captured and end up in the guerillas' future. In this future a bomb at the peace conference started a world-wide war. The Daleks travelled back in time to invade Earth a century early and had complete success. The guerillas travelled back in time to prevent the bomb by killing Styles. The Doctor makes them realise that the bomb was one of the rebels' own bombs. Returning to the present, he arranges for the delegates at the conference to be evacuated. He lures a force of Daleks and Ogrons into the peace conference venue and a guerilla, who had been stranded in this time blows up the building. This action destroys the future that had been created.
When the Doctor and Jo return to the UNIT lab, they encounter their earlier selves, as the earlier Doctor fiddles with his dematerialisation circuit.